Permutations of vowels

// Exercise: permutations of all vowels in a String

object Str {
  private def perm(base: List[Char], acc: List[Char]): List[List[Char]] = {
    if (base.size == acc.size) List(acc)
    else base filter(!acc.contains(_)) flatMap { c =>
      perm(base, c :: acc)
    }
  }

  // usage: val s = "abcdei"; vowelPerm(s, n => "aeiouyAEIOUY".contains(s(n)))
  def vowelPerm(s: String, predicate: Int => Boolean): List[String] = {
    val vowels = s.zipWithIndex.filter{t => predicate(t._2)}.map{_._1}.toList
    perm(vowels, Nil).map { p =>
      var k = -1
      val result = s.zipWithIndex.map {
        case (_, i) if predicate(i) => k+=1; p(k)
        case (c, _) => c
      }.toArray
      new String(result)
    }
  }
}

// test:
val s = "abcdei"
Str.vowelPerm(s, n => "aeiouyAEIOUY".contains(s(n))) foreach println

// output:
// ibcdea
// ebcdia
// ibcdae
// abcdie
// ebcdai
// abcdei